Single Crystalline 2D Layers with Linear Magnetoresistance and High C

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Thickness-Controlled Synthesis of CoX2 (X = S, Se, and Te) Single Crystalline 2D Layers with Linear Magnetoresistance and High Conductivity Xingguo Wang, Zhang Zhou, Peng Zhang, Shuqing Zhang, Yang Ma, Weiwei Yang, Hao Wang, Bixuan Li, Lingjia Meng, Huaning Jiang, Shiqiang Cui, Pengbo Zhai, Jing Xiao, Wei Liu, Xiaolong Zou, Lihong Bao,* and Yongji Gong*

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ABSTRACT: Two-dimensional (2D) materials especially transition metal dichalcogenides (TMDs) have drawn intensive interest owing to their plentiful properties. Some TMDs with magnetic elements (Fe, Co, Ni, etc.) are reported to be magnetic theoretically and experimentally, which undoubtedly provide a promising platform to design functional devices and study physical mechanisms. Nevertheless, plenty of theoretical TMDs remain unrealized experimentally. In addition, the governable synthesis of these kinds of TMDs with desired thickness and high crystallinity poses a tricky challenge. Here, we report a controlled preparation of CoX2 (X = S, Se, and Te) nanosheets through chemical vapor deposition. The thickness, lateral scale, and shape of the crystals show great dependence on temperature, and the thickness can be controlled from a monolayer to tens of nanometers. Magneto-transport characterization and density function theory simulation indicate that CoSe2 and CoTe2 are metallic. In addition, unsaturated and linear magnetoresistance have been × 6 × 6 observed even up to 9 T. The conductivity of CoSe2 and CoTe2 can reach 5 10 and 1.8 10 S/m, respectively, which is pretty high and even comparable with silver. These cobalt-based TMDs show great potential to work as 2D conductors and also provide a promising platform for investigating their magnetic properties.

1. INTRODUCTION predicted by theoretical calculation, till date have hardly been 34 The family of two-dimensional (2D) materials such as explored on the scale of atomic layers experimentally, and the 1,2 3 4,5 6 7 scalable preparation of these TMDs with controllable layer graphene, h-BN, black phosphorus, CrI3, FePS3, and

Downloaded via INST OF PHYSICS on November 7, 2020 at 07:23:11 (UTC). transition metal dichalcogenides (TMDs) has shown exciting numbers and high quality still remains a serious challenge. potentials in various aspects, including both scientific research For instance, layered TMD CoX2 (X = S, Se, and Te) is (superconductivity, charge density wave, magnetism, topo- composed of hexagonal unit cells, where each Co atom is 8−14 bonded with six X (chalcogen) atoms. Because of the existence See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. logical properties, etc.) and practical applications (elec- − tronics, optoelectronics, catalysis, energy storage, etc.).15 25 of cobalt elements, CoX2 is possible to behave as a magnet Among these, TMDs have drawn intensive attention because with atomic thickness. A few studies have been conducted on of their abundant types and thickness-dependent proper- the preparation and application of CoX2. For example, a ties.26,27 For example, the superconducting transition temper- nonlayered material CoS2 is reported as ferromagnetic metal with the Curie temperature TC of 124 K, and CoSe2 with the ature of NbSe2 nanosheets is strongly dependent on their layer 28 ff − − pyrite-type structure is regarded as the controversial Pauli numbers. Three di erent charge density wave phases have 35−37 29 paramagnet or the antiferromagnet. Some initial efforts been observed in TaS2. Linear magnetoresistance (MR) and topological superconductivity have also been discovered in have also been made on CoTe2 in two dimensions recently. 30 Duan and co-workers reported the layer-controlled growth of WTe2 nanosheets. Recently, some TMDs with magnetic elements such as CrSe2, CrTe2, VTe2, VTe2, and so forth have been predicted theoretically or proved experimentally to have Received: October 28, 2019 − some magnetic properties,31 33 which can be served as a Revised: March 4, 2020 powerful platform to design functional devices and study Published: March 4, 2020 physical magnetic mechanisms. Generally, plenty of funda- mental physical phenomena and potential applications can be realized in 2D TMDs. However, some new kinds of TMDs,

Figure 1. Preparation and morphology of CoX2 (X = S, Se, and Te). (a) Schematic illustration of the growth process. Mixture of Co3O4 and NaCl are used as Co precursors which can react with S or Se or Te gas at a certain temperature and atmosphere. (b−d) OM images and height proﬁles of ultrathin CoS2, CoSe2, and CoTe2 nanoplates on SiO2/Si. (e) OM image of the large view of CoTe2 with uniform thickness. (f) Diagrams of the smallest thickness of CoTe2 nanosheets plotted as a function of the growth temperature. (g) Evolution of the largest lateral scale of CoTe2 nanosheets with the growth temperature.

CoTe2 and CoSe and studied the electric properties of the the conductivity of silver showing promising application resulting nanosheets.38,39 Wang and co-workers found tunable potential as 2D conductors. According to the magneto- ff magnetic properties of CoTe2 treated with di erent concen- transport and superconducting quantum interference device 40 trations of NaOH. However, high-quality ultrathin-layered (SQUID) test, both CoSe2 and CoTe2 seem likely to be CoX2 (X = S and Se) has not been synthesized yet. Moreover, paramagnetic in nature. Density function theory (DFT) is CoX2 nanosheets with well-controlled thickness down to the further used to simulate the band structures of CoX2, further monolayer are desirable to explore the layer-dependent confirming their metallic nature. Altogether, the studies magnetic properties of these materials. illustrate a controllable way to synthesize 2D CoX2, which Herein, we report a chemical vapor deposition (CVD) may provide a promising opportunity for studying the approach to synthesize few-layer CoX2 nanosheets with the properties of 2D CoX2 with various layers, such as electronic, controlled thickness on SiO2/Si substrates by tuning the magnetic, catalytic, and other properties. growth temperatures. By using monolayer MoSe2 as substrates (monolayer MoSe2 developed on SiO2/Si substrates in 2. RESULTS AND DISCUSSION fi advance), a single-layer CoX2 can be synthesized for the rst The CoX2 (X = S, Se, and Te) nanosheets on 285 nm SiO2/Si time. In addition, the growth temperature plays an important substrates are synthesized using a traditional CVD system, as role in the evolution of the lateral size and shape. Optical shown in Figure 1a, with the mixture of Co3O4 (0.1 g) and microscopy (OM) images illustrate the morphologies of the as- NaCl (0.03 g) as the Co source while different chalcogen grown CoX2 nanosheets, mostly displaying a hexagonal or element powders as X precursors. Although the growth time ∼ triangular shape with the lateral domain size ranging from 5 and the flow of carrier gas can also cause a certain influence on ∼ μ to 100 m. Atomic force microscopy (AFM) studies reveal the thickness, which was reported in other TMDs such as the thickness of the nanosheets varying from 1.18 to ∼20 nm. 41 ff NiTe2, the thickness of MX2 can still be tuned e ectively by X-ray photoemission spectroscopy (XPS) is carried out to controlling the growth temperature in this study. More details analyze chemical states, and Auger electron spectroscopy of the sample synthesis are described in the Methods section. − (AES) mapping indicates the compositional homogeneity of The OM images, as shown in Figure 1b d, indicate that CoX2 the as-grown CoX2 samples. The crystal structure and high (X = S, Se, and Te) nanosheets exhibit hexagonal or triangular crystalline quality are confirmed by high-resolution trans- shapes with various lateral size. The AFM results demonstrate mission electron microscopy (HRTEM) and selected-area that the typical thickness of CoS2, CoSe2, and CoTe2 is about electron diffraction (SAED) characterizations. Magneto-trans- 7.2, 5.1, and 3.9 nm, respectively. Figure 1e shows the OM port studies show that CoX2 (X = Se and Te) single crystals image of the uniform CoTe2 nanosheets in a large view. exhibit metallic behaviors. Additionally, unsaturated linear MR In order to achieve controllable synthesis of these 2D (LMR) up to 9 T occurs in both CoSe2 and CoTe2 nanosheets. materials, we take CoTe2 as an example and conduct ff Meanwhile, both CoSe2 and CoTe2 with various thicknesses systematic studies to investigate the e ect of growth temper- 6 fl show outstanding conductivity up to 10 S/m, which is close to ature on the thickness and lateral size. With the Ar/H2 ow

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Figure 2. Characterizations of chemical states and compositional homogeneity. (a and e) XPS spectra of Co 2p from CoSe2 and CoTe2 nanosheets. (b and f) XPS spectra of Se 3d and Te 3d. Indexed peaks are formed by CoSe2 or CoTe2, while the others come from some contaminations during the CVD process. (c,d,g, and h) AES mapping spectra of CoSe2 and CoTe2 nanosheets. ° rate constant at 50 sccm in which 10% of the gas is H2 and the to 780 C. This phenomenon can be explained by the local time of deposition process constant is 3 min, the nanosheets changes in the Co/Te ratio at different temperatures, which ff ° 45 produced at di erent temperatures from 720 to 800 C show a has been discussed in the CVD growth of MoS2. clear thickness evolution, as shown in Figure 1f. With Although the growth temperature and deposition time can ff increasing growth temperatures, thicker CoTe2 nanosheets e ectively tune the thickness of CoX2, it still remains a are grown. However, once the temperature is lower than 740 challenge to obtain a single-layer CoX2 on the SiO2/Si °C, the lateral size of the nanosheets becomes smaller while the substrate. However, as reported in the literature, obtaining thickness remains almost unchanged. In addition, the single-layer samples is quite significant because the optical, revolution of the lateral size of CoTe2 with the temperature electrical, and magnetic properties of 2D materials exhibit ranging from 720 to 800 °C is also studied when the strong thickness dependence.6,46 We speculate that a single- fl deposition time and ow rate remain constant. It is obvious layer CoX2 may be unstable on the SiO2/Si substrate because that the largest lateral size increases from several to tens of of the high interface energy. Hence, when MoSe2/SiO2/Si is micrometers with the temperature increasing from 720 to 780 designed as the substrate, it turns out to obtain single-layer ° C(Figure S2). When the temperature is further raised to 800 samples. The OM image of thin CoSe2 deposited on MoSe2/ ° fl ff C, the largest lateral size slightly uctuates (Figure 1g). These SiO2/Si nanosheets is shown in Figure S4e, where di erent data reflect that the material preferably grows in both vertical color contrasts indicate different phases. The AFM height fi and lateral directions with the increasing temperature. pro le image in Figure S4f shows that the thinnest CoSe2 Thermodynamics and kinetics are the two key factors to nanosheets produced on MoSe2/SiO2/Si is 1.18 nm, indicating control the growth of crystals, which can be used to explain the a single layer which probably results from smaller nucleation aforementioned phenomena. As explained in the previous and growth energy on MoSe2/SiO2/Si. Consequently, the paper, vertical growth of 2D materials is resulted from the single-layer nanosheets of the other CoX2 (X = S and Se) may thermodynamic product because of the van der Waals force be obtained in the same way. between adjacent layers, and in-plane growth of 2D materials is Totally, the suitable growth temperature for CoTe2 may caused by the kinetic product because of the lower nucleation occur between 740 and 760 °C, during which a relatively thin barrier when putting atoms to the edges with unsaturated (∼4 nm) and an averagely large lateral domain (∼20 μm) 42−44 bonds. It is well known that the thermodynamic product CoTe2 nanosheets can be obtained. CoS2 and CoSe2 is preferred at high temperature and the kinetic product is nanosheets with different layer numbers can be synthesized preferred at low temperature. In our case, when the by a similar route. Notably, CoX2 crystals can degenerate badly temperature is below 740 °C, the reaction is controlled by in air after some days while the oxidation of the samples kinetics. In this situation, when increasing the temperature, the happens slowly in an evacuated bag, as shown in Figure S5. lateral size of the sample becomes larger because of more vapor Thus, in order to avoid possible oxidation of the samples, all from the precursors. When the temperature is higher than 780 the characterizations are conducted as soon as possible. Raman °C, the reaction is dominated by thermodynamics. Thus, a studies together with OM images of the as-grown ultrathin higher temperature will result in a thicker sample rather than a nanoplates are shown in Figures S7 and S1. larger size. With the temperature ranging from 740 to 780 °C, The chemical states and the compositional homogeneity of the competition between thermodynamics and kinetics leads to the resulting CoX2 (X = Se and Te) samples were analyzed by the increased thickness and size when increasing the XPS and AES mapping, respectively. The survey-scanning XPS fi temperature. spectra for CoSe2 and CoTe2, as shown in Figure S8, con rm Meanwhile, there is morphological transition when the the presence of Co, Se, and Te. Figure 2a,e demonstrates the growth temperature changes (Figure S3). It is clear to see that Co 2p core-level spectra of CoSe2 and CoTe2. Two peaks at 2+ 2+ the ratio of triangular nanosheets decreases while hexagonal 780.0 and 796.8 eV correspond to Co 2p3/2 and Co 2p1/2 of crystals dominate when the temperature is increased from 740 CoSe2, and the peaks located at 778.4 and 793.7 eV may result

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Figure 3. Crystal structure of CoX2 (X = Se and Te) determined by TEM. (a and e) Morphology images of the transferred CoSe2 and CoTe2 fi ff nanosheets on a copper mesh with the carbon lm. (b and f). SAED patterns of the CoSe2 and CoTe2 nanosheets. Di raction plots in the two fi pictures show a similar 6-fold symmetry, con rming that CoSe2 and CoTe2 nanosheets share similar crystal structures. (c and g) HRTEM images of ff the CoSe2 and CoTe2 nanoplates. Measured lattice spacings of CoSe2 are smaller than those of CoTe2, consistent with the di erence between the radius of Se and Te atoms. (d and h) Atomic structure of the CoSe2- and CoTe2-layered nanosheets. Pink, green, and brown spheres represent Co, Se, and Te atoms, respectively.

Figure 4. Magneto-transport properties of CoSe2 and CoTe2 nanosheet devices. (a) Illustrations of typical Hall-bar devices of CoX2 crystals. (b and Δρ ρ fi ρ − ρ ρ c) Temperature dependence of the resistivity R for CoSe2 and CoTe2 from 300 to 2 K. (d and f) The MR ( / 0), de ned as [ (H) (0)]/ (0) × ff − fi 100%, of these two nanosheets characterized at di erent temperatures with a 0 9 T magnetic eld. Temperature for CoSe2 is set at 2, 4, 8, 15, − fi and 25 K, while for CoTe2, temperature is set at 2, 6, 10, and 20 K. (e and g) Hall resistance (Rxy) curves of the devices with a 0 9 T magnetic eld ff at di erent temperatures. (h) Comparison of the conductivity of CoX2 nanosheets at room temperature with various conductive 2D materials, bulk ff materials, and conventional metals. (i) Comparison of MR at 2 K and conductivity of CoTe2 nanosheets with di erent thicknesses.

2+ from Co 2p3/2 and Co 2p1/2 of CoO1−x, respectively, which standable that the binding energies for Co 2p1/2 and 2p3/2 of ∼ shows similar chemical states with Co. The higher binding CoSe2 are 1 eV higher than that of CoTe2 because the Se energies 784.1 and 802.2 eV can be assigned from Co3O4. atom has stronger electronegativity than the Te atom. As ﬁ Meanwhile, as shown in Figure 2e, Co 2p3/2 signals at 778.0 shown in Figure 2b, the Se 3d core level spectrum can be tted and 780.1 eV are attributed to CoO1−x and CoTe2, with Se 3d5/2 (54.2 eV) and Se 3d3/2 (55.0 eV) peaks, which is 49 respectively. Co 2p1/2 signals at 793.0 and 798.5 eV result consistent with the reported CoSe2. Figure 2f indicates three from CoO1−x and CoTe2, respectively. Both Co 2p spectra sets of peaks corresponding to the Te 3d5/2 and Te 3d3/2 of suggest that CoSe2 and CoTe2 samples can be oxidized in air, CoTe2, Te(0), and TeO2, respectively, among which CoTe2 is ﬀ generating di erent cobalt oxides with various valence states indexed to Te 3d5/2 (572.1 eV) and Te 3d3/2 (582.5 eV). The 47,48 such as CoO1−x and Co3O4. Additionally, it is under- signals of Te 3d5/2 (573.2 eV), Te 3d3/2 (583.5 eV), Te 3d5/2

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− Figure 5. (a c) Calculated partial band structures and density of states (DOS) of CoX2 (X = S, Se, and Te) by DFT, respectively. Both the energy band and the DOS are generated from Co atoms and chalcogen atoms, and the purple parts represent contribution from Co, while the yellow, green, and orange parts are from S, Se, and Te, respectively. Fermi energy (EF) is set at 0 eV. ff (575.9 eV), and Te 3d3/2 (586.3 eV) come from Te(0) and phase because of similar di raction plots between CoTe2 and fi 50,51 38 TeO2 signi cantly. Given that the oxidation state of the CoSe2. 52 transition metal Co is +2. Hence, Te and Se in CoSe2 and The transport properties of CoSe2 and CoTe2 were probed 2− 2− ff CoTe2 are formulated to be Se2 and Te2 except Te(0) in with Hall e ect measurements on the single-crystal samples. In 4+ elemental tellurium and Te in TeO2. Interestingly, there are order to reduce oxidation, the samples for device fabrication three pairs of peaks, as shown in Figure 2a, but only two pairs were coated by poly(methyl methacrylate) immediately after in 2e, which may indicate the serious oxidation of Co in CoSe2. being taken out from the quartz tube. Figure 4a shows the In contrast, there are three pairs of peaks in 2f for Te (CoTe , typical Hall-bar device of CoX2 crystals, where the applied 2 fi Te, and TeO2) but one in 2b for Se, which may be because of magnetic eld is perpendicular to the sample. Figure 4b,c the oxidation of Te. Thus, it can be concluded that one layer of shows the longitudinal resistance Rxx from 2 to 300 K on the CoSe and CoTe single crystals. The resistance of CoSe and Te is formed on the surface of CoTe2 as the protective layer 2 2 2 because of its low vapor pressure when compared to Se, which CoTe2 nanosheets decreases as the temperature is reduced − can further prove that the high oxidation states of Co in CoSe from 300 to 2 K resulting from the decrease of the electron 2 − resulted from the absence of protective layers. AES mapping phonon interaction and electron electron scattering, which images clearly show the spatial distribution of the Co, Se, and indicatesthattheelectrictransportpropertiesofboth nanosheets behave like a metal.53,54 Figure 4d,f demonstrates Te elements in the hexagonal CoSe2 and CoTe2 nanosheets, as − shown in Figure 2c,d,g,h. The uniform color contrasts illustrate the MR in these two kinds of materials. When the 0 9T magnetic field is applied perpendicular to the direction of the the homogeneity of elements distribution in the samples. fl fi However, it is worth noting that the brighter edges in the Te current ow, the eld dependence of MR indicates a linear mapping image may be because of the enrichment of Te in the positive relationship in both nanosheets, which can be remarked as LMR. As for the gapless CoSe2 and CoTe2, the edges of the CoTe2 ultrathin nanoplates, which may be because of some excess Te attached to the samples. uneven distribution of conductivity or disorder in the system may result in LMR.55 All the MR values decrease with To further study the chemical composition and crystal increasing temperatures, which may be directly consequenced structure of the prepared CoSe and CoTe nanosheets, 2 2 as the huge rise in resistivity as the temperature rises. The Hall energy-dispersive spectroscopy (EDS), SAED, and HRTEM resistivity R of CoSe nanosheets at different temperatures analyses were carried out. EDS, as shown in Figure S9, xy 2 (2−25 K) exhibits a linear dependency with the applied field, illustrates that the atomic ratios of Co/Se and Co/Te in the as shown in Figure 4e, which is also shown in the case of transferred nanosheets are 1:2.11 and 1:2.09, respectively, CoTe2 nanosheets in Figure 4g. According to the linear which is very close to 1:2 within the error range, showing good fi dependency between Rxy and the magnetic eld, the Hall agreement with the expected stoichiometric ratio. Figure 3a,e coefficient for those two devices can be derived. Then, taking 6 shows the magnified view of the hexagonal nanosheets on and 13 nm as the thickness of CoSe2 and CoTe2, respectively, TEM microgrids. Figure 3b,f indicates the SAED patterns of we can calculate the carrier density as high as 9.2 × 1023 cm−3 − the CoSe2 and CoTe2. There is only one single set of similar × 22 3 ff for CoSe2 and 5.4 10 cm for CoTe2. It is worth noting di raction spots with 6-fold symmetry for both of them, that both devices maintain very low resistance from 300 to 2 K. further proving that both of the synthesized nanosheets are The electrical conductivity of CoSe2 and CoTe2 can reach as monocrystalline with a similar structure. The obvious lattice high as 5 × 106 and 1.8 × 106 S/m, respectively, which is even distances of 0.184 and 0.321 nm can be observed from the comparable with that of the conventional silver. Compared HRTEM image of the CoSe ultrathin nanoplate, correspond- 56 57 58 2 with graphene, ReSe2, liquid-exfoliate VSe2 (E-VSe2), ing to the (110) and (100) planes of the CoSe crystal 59 60 61 2 CVD growth of VSe2 (CVD-VSe2), thin NbAs, Bismuth, 62 63 structure, as shown in Figure 3c. Similarly, the measured WTe2, bulk MX2 (M = Nb and Ta; X = Se and Te), 46 64 65 interplane spacings of 0.197 and 0.341 nm, as shown in Figure PtSe2, PtTe2, and Ti3C2, as shown in Figure 4h, the 3g, are consistent with the (110) and (100) planes of the electrical conductivity of CoX2 is outstanding among the state- CoTe2. In addition, it is reasonable that the lattice parameters of-the-art conductive 2D materials, indicating promising of CoTe2 are larger than those of CoSe2 because the radius of application potential as good 2D conductors. So as to study the Te atom is larger than that of the Se atom. Figure 3d,h the thickness-dependent magneto-transport properties of shows the stick-and-ball crystal structure model of CoSe2 and CoX2, CoTe2 is selected for instance, and three CoTe2 devices CoTe2, which displays 6-fold symmetry. In the crystal of CoX2, with different thicknesses were nanofabricated. All devices each Co atom is surrounded by six X atoms in the octahedron display good contact and excellent conductivity during the test coordination type. Both CoTe2 and CoSe2 should be in 1T temperature range, as shown in Figure S11. Additionally, all

2325 https://dx.doi.org/10.1021/acs.chemmater.9b04416 Chem. Mater. 2020, 32, 2321−2329 Chemistry of Materials pubs.acs.org/cm Article devices display LMR at different temperatures, but the value of for CoSe2 and CoS2, Se and S can be placed at the end of the quartz LMR at 2 K becomes slightly lower with the increasing tube furnace with approximate temperatures 200 and 130 °C, respectively. Then, the CVD system was purged with ultrahigh purity thickness, as shown in Figure 4i. Meanwhile, the conductivity ∼ of the samples with various thicknesses remains as high as 106 argon (Ar) gas ( 99.999%) to remove oxygen and moisture from the quartz tube. Afterward, the furnace was heated up to 750 °C for 18 S/m, showing no obvious thickness dependence. min and held at 740 °C for 3 min with the flow of mixed gases of 5 The SQUID test was further conducted to explore other sccm H2 and 45 sccm Ar under atmospheric pressure. After the magnetic properties of CoX2 crystals, as shown in Figure S12. reaction, the furnace was naturally cooled to room temperature in the − Although there is a small loop in the M H curves of CoTe2 flow of 100 sccm Ar. With the temperature of the source tuned from ° ff and CoSe2, it may come from the oxidation products or the 720 to 800 C, di erent thicknesses of CoSe2 and CoTe2 nanosheets impurities, which needs to be further explored in detail in could be produced on the SiO2/Si substrates. Meanwhile, MoSe2/ SiO /Si substrates were placed above the powder to obtain single- future work. It is safe to conclude that both CoSe2 and CoTe2 2 are paramagnetic in nature according to the magneto-transport layer CoX2. and the M−T curves. 4.2. Sample Characterization. The morphology and thickness of the as-grown CoSe2 and CoTe2 nanosheets were characterized by an The simulated band structures of CoX2 (X = S, Se, and Te), − optical microscope (CX44, OLYMPUS) and an atomic force both with and without spin orbit coupling interaction, are microscope (ICON, VEECO/Bruker). SEM (Zeiss, Germany) was shown in Figure S13. All of these three materials are metallic, used to directly observe the morphology of the obtained nanosheets − which is in agreement with the R T curve of the metallic on the SiO2/Si substrates as well. The chemical composition, CoSe2 nanosheets and CoTe2, as shown in Figure 4. For all of chemical states, and compositional homogeneity of the ultrathin these band diagrams, the minimum of the lowest unoccupied nanosheets were determined by Raman spectra and mapping (DXRxi, band is along K → Γ, while the maximum of the partially Thermo) with a 532 nm laser as the excitation source, XPS (250Xi, fi occupied band locates at Γ point, which indicates the similarity Thermo Scienti c ESCALAB), and AES mapping (PHI710, ULVAC- PHI). The crystal structures of the transferred CoSe2 and CoTe2 among CoX2. The projected energy band structures and DOS − nanosheets onto the copper grids were examined using a JEM-2100F, of CoX2, as shown in Figure 5a c, demonstrate that chalcogen fi operating at 200 kV and equipped with an EDS system. atoms play a signi cant role in the position of the top of the 4.3. Device Fabrication and Characterization. The CoSe and valence bands. Additionally, the position of the maximum of 2 CoTe2 Hall devices were fabricated by a standard e-beam lithography the partially occupied bands becomes higher from S to Te. The technique followed by the deposition of Ti/Au for electrical contact bottom of the lowest unoccupied bands for CoX2 is (Ti: 10 nm and Au: 40 nm). The Hall measurement was performed contributed by Co and chalcogen atoms together, which gets with the six terminal Hall bar structure in a vacuum chamber with the pressure lower than 10−5 Torr using a physical property measurement closer to the EF from S to Te. system (Quantum design). The conductivity is obtained using the 3. CONCLUSIONS four-terminal measurement. During characterization, the magnetic field can be loaded as high as 9 T while the temperature can be cooled In summary, we successfully synthesized ultrathin CoX2 (X = to 2 K. S, Se, and Te) with tunable thickness ranging from 1.18 nm 4.4. Calculation Method. All the DFT calculations were (single layer) to tens of nanometers. In addition, the performed with the Vienna Ab initio Simulation Package (VASP)66,67 using the Perdew−Burke−Ernzerhof functional within deposition temperature is found to be the key factor to 68 control the thickness, lateral scale, and shape evolution of the generalized gradient approximation. The optimized lattice constants were 3.23, 3.37, and 3.65 Å for monolayer CoS2, CoSe2, and CoX2 crystals. The resulting CoX2 (X = Se and Te) nanosheets ffi CoTe2, respectively. A su ciently larger distance of c = 20 Å along the are characterized by XPS, AES, and TEM, displaying out-of-plane direction was selected to simulate isolated monolayer chemically uniform and high-quality single-crystalline nano- properties. The uniform Monkhorst−Pack k mesh of 17 × 17 × 1 was plates. Magneto-transport studies of the CoSe2 and CoTe2 adopted for integration over the Brillouin zone. The kinetic energy nanosheets show excellent conductivity similar to that of silver cutoff for the plane-wave basis was set to 500 eV. All the structures and LMR up to 9 T at low temperature, indicating their were fully relaxed until the force on each atom was less than 0.01 eV/ potential electrical and magnetic applications. DFT simulations Å. fi further con rm the metallic property of CoSe2 and CoTe2. ■ ASSOCIATED CONTENT Generally, the thickness-controlled CoX2 single-crystalline nanoplates with superior conductivity will provide an out- *sı Supporting Information standing candidate for investigating 2D magnetism, electronics, The Supporting Information is available free of charge at and other low-dimensional devices and phenomena. https://pubs.acs.org/doi/10.1021/acs.chemmater.9b04416. OM and AFM images of resulting CoX nanoplates with 4. METHODS 2 various thicknesses, XPS wide-scanning spectra of CoSe2 4.1. Synthesis of CoSe and CoTe Nanosheets. 2 2 CoSe2 and and CoTe2 nanosheets, EDS images of CoSe2 and CoTe nanosheets (2D) were deposited on SiO /Si substrates using a 2 2 CoTe2 nanosheets indicating content of each element, CVD system under atmospheric pressure. Cobaltosic oxide (0.1 g) EDS mapping of CoS shows uniformity of element (99.7%, Alfa) with sodium chloride (0.03 g) (99.85%, Acros) to 2 distribution, Raman studies of CoX2 nanoplates with reduce the melting point of the oxide was placed in a ceramic boat, ff di erent thicknesses, Magneto-transport dates of CoSe2 which is in the center of the heating zone with a piece of the SiO2/Si ff substrate facing downward above the mixture powder. Sulfur (1 with di erent thicknesses, SQUID results of CoX2 g)(99.5%, Acros)/selenium (0.1 g) (99.999%, Acros)/tellurium crystals, and DFT simulation of CoX2 crystals (PDF) power (0.1 g) (99.999%, Acros) in a ceramic boat was placed in the upstream zone, where the temperature would be adjusted by ■ AUTHOR INFORMATION moving the boat when the center hot zone of the furnace is set at 740 °C. As a consequence of the vaporization temperature of Te being Corresponding Authors higher than that of Se and S, the boat of Te powder should be moved Lihong Bao − Institute of Physics & University of Chinese ° near the high-temperature zone (420 C) to synthesize CoTe2, while Academy of Sciences, Chinese Academy of Sciences, Beijing

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100190, P. R. China; orcid.org/0000-0002-2942-892X; Innovative and Entrepreneurial Research Team Program Email: [email protected] (grant no. 2017ZT07C341), the Economic, Trade, and Yongji Gong − School of Materials Science and Engineering, Information Commission of Shenzhen Municipality for the Beihang University, Beijing 100191, P. R. China; orcid.org/ “2017 Graphene Manufacturing Innovation Center Project 0000-0003-1432-6813; Email: [email protected] (no. 201901171523).” Authors ■ ABBREVIATIONS Xingguo Wang − School of Materials Science and Engineering, 2D, two-dimensional; TMDs, transition metal dichalcogenides; Beihang University, Beijing 100191, P. R. China CVD, chemical vapor deposition; LMR, linear magneto- Zhang Zhou − Institute of Physics & University of Chinese resistance; DFT, density function theory; OM, optical Academy of Sciences, Chinese Academy of Sciences, Beijing microscopy; AFM, atomic force microscopy; XPS, X-ray 100190, P. R. China photoemission spectroscopy; AES, Auger electron spectrosco- Peng Zhang − School of Materials Science and Engineering, py; HRTEM, high-resolution transmission electron micros- Beihang University, Beijing 100191, P. R. China copy; SAED, selected-area electron diffraction; EDS, energy- Shuqing Zhang − Shenzhen Geim Graphene Center and Low- dispersive spectroscopy; SQUID, superconducting quantum Dimensional Materials and Devices Laboratory, Tsinghua- interference device Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, P. R. China; orcid.org/0000-0003- REFERENCES 3989-4585 ■ Yang Ma − School of Materials Science and Engineering, Beihang (1) Huang, S.; Dakhchoune, M.; Luo, W.; Oveisi, E.; He, G.; Rezaei, University, Beijing 100191, P. R. China M.; Zhao, J.; Alexander, D. T. L.; Zuttel, A.; Strano, M. S.; Agrawal, K. Weiwei Yang − School of Materials Science and Engineering, V. Single-layer graphene membranes by crack-free transfer for gas mixture separation. Nat. Commun. 2018, 9, 2632. Beihang University, Beijing 100191, P. R. China − & (2) Cao, Y.; Fatemi, V.; Demir, A.; Fang, S.; Tomarken, S. L.; Luo, J. Hao Wang Institute of Physics University of Chinese Y.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Academy of Sciences, Chinese Academy of Sciences, Beijing Ashoori, R. C.; Jarillo-Herrero, P. Correlated insulator behaviour at 100190, P. R. China half-filling in magic-angle graphene superlattices. Nature 2018, 556, Bixuan Li − School of Materials Science and Engineering, 80−84. Beihang University, Beijing 100191, P. R. China (3) Tian, H.; Khanaki, A.; Das, P.; Zheng, R.; Cui, Z.; He, Y.; Shi, Lingjia Meng − School of Materials Science and Engineering, W.; Xu, Z.; Lake, R.; Liu, J. Role of Carbon Interstitials in Transition Beihang University, Beijing 100191, P. R. China Metal Substrates on Controllable Synthesis of High-Quality Large- − Area Two-Dimensional Hexagonal Boron Nitride Layers. Nano Lett. 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Impact of Thickness on Contact Wei Liu − School of Materials Science and Engineering, Beihang Issues for Pinning Effect in Black Phosphorus Field-Effect Transistors. University, Beijing 100191, P. R. China Adv. Funct. Mater. 2018, 28, 1801398. Xiaolong Zou − Shenzhen Geim Graphene Center and Low- (6) Huang, B.; Clark, G.; Navarro-Moratalla, E.; Klein, D. R.; Cheng, Dimensional Materials and Devices Laboratory, Tsinghua- R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu, X. Layer-dependent Berkeley Shenzhen Institute (TBSI), Tsinghua University, ferromagnetism in a van der Waals crystal down to the monolayer Shenzhen 518055, P. R. China; orcid.org/0000-0002- limit. Nature 2017, 546, 270−273. 3987-6865 (7) Cheng, Z.; Shifa, T. A.; Wang, F.; Gao, Y.; He, P.; Zhang, K.; Complete contact information is available at: Jiang, C.; Liu, Q.; He, J. High-Yield Production of Monolayer FePS3 https://pubs.acs.org/10.1021/acs.chemmater.9b04416 Quantum Sheets via Chemical Exfoliation for Efficient Photocatalytic Hydrogen Evolution. Adv. Mater. 2018, 30, 1707433. (8) Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Author Contributions Kaxiras, E.; Jarillo-Herrero, P. Unconventional superconductivity in X.W., Z.Z., and P.Z. contributed equally. The manuscript was magic-angle graphene superlattices. Nature 2018, 556,43−50. written through the contributions of all authors. All authors (9) Fei, Z.; Huang, B.; Malinowski, P.; Wang, W.; Song, T.; Sanchez, have given approval to the final version of the manuscript. J.; Yao, W.; Xiao, D.; Zhu, X.; May, A. F.; Wu, W.; Cobden, D. H.; Chu, J.-H.; Xu, X. Two-dimensional itinerant ferromagnetism in Notes − fi atomically thin Fe3GeTe2. Nat. Mater. 2018, 17, 778 782. The authors declare no competing nancial interest. (10) Yamada, K.; Souma, S.; Yamauchi, K.; Shimamura, N.; Sugawara, K.; Trang, C. X.; Oguchi, T.; Ueno, K.; Takahashi, T.; ■ ACKNOWLEDGMENTS Sato, T. Ultrathin Bismuth Film on 1T-TaS2: Structural Transition The authors acknowledge support from the Key Technologies and Charge-Density-Wave Proximity Effect. Nano Lett. 2018, 18, 3235−3240. Research and Development Program of China (grant nos. (11) Bahramy, M. S.; Clark, O. J.; Yang, B.-J.; Feng, J.; Bawden, L.; 2018YFA0306900 and 2016YFA0202300) and the National Riley, J. M.; Markovic,́ I.; Mazzola, F.; Sunko, V.; Biswas, D.; Cooil, S. Natural Science Foundation of China (51872012). The P.; Jorge, M.; Wells, J. W.; Leandersson, M.; Balasubramanian, T.; theoretical part was supported by the Shenzhen Basic Research Fujii, J.; Vobornik, I.; Rault, J. E.; Kim, T. K.; Hoesch, M.; Okawa, K.; Projects (no. JCYJ20170407155608882), the Guangdong Asakawa, M.; Sasagawa, T.; Eknapakul, T.; Meevasana, W.; King, P.

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